In 2003 Pomerening et al. provided strong evidence for this hypothesis by demonstrating hysteresis and bistability in the activation of Cdk1 in the cytoplasmic extracts of Xenopus oocytes.
The pulse of G1/S cyclins causes CLN3-Cdk1 to activate Cln1/2, (Start point (yeast)) as well as Clb5/6 at the initiation of S-phase.